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    <title>Document</title>
    <script>
        //案例1：结果是几?
        function f1() {
            var num = 123;
            function f2() {
                console.log(num);//站在目标出发，一层一层的往外查找
            }
            f2();
        }
        var num = 456;
        f1();//123

        // 案例2:结果是几?
        var a = 1;
        function fn1() {
            var a = 2;
            var b = '22';
            fn2();
            function fn2() {
                var a = 3;
                fn3();
                function fn3() {
                    var a = 4;
                    console.log(a);//4
                    console.log(b);//'22'
                }
            }
        }
        fn1();
    </script>
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